This invention relates generally to radiological well logging methods and apparatus for investigating the subsurface earth formations traversed by a borehole and, more particularly, to an improved method and apparatus for high frequency pulsed neutron gamma ray logging wherein gamma rays resulting from neutron inelastic scatterings and thermal neutron capture are selectively detected and the spectral distribution of the gamma rays is determined.
A number of well logging techniques in use come under a general classification of neutron induced radiological well logging. In their basic form, these techniques involve irradiating subsurface earth formations with neutrons and then determining the effect of select formation constituents on the neutrons by measuring either gamma rays produced by inelastically scattered neutrons or the gamma rays resulting from thermal neutron capture. The detection of these radioactive signals returning to the borehole can provide information as to the porosity, lithology and presence or absence of hydrocarbons within the formation. One property of subsurface formations of particular interest is porosity, which in rocks, is space not occupied by solid material, expressed in percent of bulk formation volume. In subsurface formations this pore space is ordinarily occupied by fluids which are hydrogenous in composition and it is well known that hydrogen has a significant capture cross section for slow neutrons. Thus, a measurement of the slow neutrons emanating from irradiated formations or a measurement of the number of gamma rays produced by thermal neutron capture reactions will furnish qualitative indications of hydrogeneity.
In the prior art it has been proposed to irradiate subsurface earth formations with a source of neutrons and measure the resulting neutron population that returns to the borehole at a preselected distance from the source. An example of such prior art instrumentation for determining porosity is described in U.S. Pat. No. 3,621,255 to R. J. Schwartz, where the neutron population returning to the borehole is sampled by a pair of neutron detectors spaced at different distances on the longitudinal axis from the neutron source. Due to the necessity of providing large counting rates, so that statistical deviation will be minimized, the source-detector spacings must be shorter than would be desired. As a result of this short detector spacing, the counting rates at the two detectors are affected, unequally, by changes in porosity. Thus, these instruments only provide a measurement related to porosity and not a measurement of true porosity. In addition, extreme care must be taken in matching detectors and discriminator level settings to provide signal validity. As a result of these problems this instrumentation lacks calibration stability needed for true porosity measurement.
Accordingly, it has been proven difficult to establish a measurement of true formation porosity. While some prior art methods and apparatus have functioned well in certain conditions, no induced gamma ray logging system has provided a formation porosity measurement substantially independent of chlorine content of the borehole and formation.